1. Field of the Invention
This invention relates to a method for fabrication of an n-channel MIS (Metal-Insulator-Semiconductor) device.
2. Description of the Prior Art
A typical type of conventional n-channel MIS device is fabricated as follows. A film of SiO.sub.2 is formed on the surface of a p-type semiconductor substrate through thermal oxidation and the SiO.sub.2 film is partially removed on the parts of the surface of the substrate where source, drain and gate contacts are to be provided. A new thin film of SiO.sub.2 is formed on the substrate through thermal oxidation and a polycrystalline silicon film is formed on the second SiO.sub.2 film through CVD (Chemical Vapor Deposition) method. The polycrystalline silicon film is selectively removed, leaving the parts thereof serving as a gate and other desired areas. The SiO.sub.2 film is selectively removed to form windows through which source and drain regions are to be formed. By using the remaining SiO.sub.2 and polycrystalline silicon films as a mask, phosphorus (P) as an n-type impurity is diffused into the exposed surface of the substrate to form source and drain regions. Through the CVD method, a PSG (glass including phosphoric oxide, called phospho-silicate glass) film is formed on the entire upper surface of the resultant substrate. In the phosphosilicate glass film are formed windows for forming source, drain and gate contacts. Aluminum (Al) is vapor-deposited on the upper surface of the substrate and the vapor-deposited Al film is selectively removed to complete Al contacts connected respectively with the source region, drain region and gate.
However, the n-channel MIS device fabricated according to the above-described method will raise the following problems.
Those surfaces of the source and drain regions in the semiconductor substrate which are in direct contact with the Al contacts tend to become p-type layers since Al diffuses into the surfaces during the process of forming the Al contacts. Accordingly, in the case where the surface impurity concentration in the source and the drain regions which are of n-type is comparatively low, i.e. below 5 .times. 10.sup.20 cm.sup.-3, the change of the source and drain regions from n-type to p-type regions gives rise to the characteristic of a diode, which will cause in turn the degradation of reliability of the resultant device and the reduction of the yield in production.
Moreover, in the case of fabricating an MIS device in which the channel length, i.e. the distance between the source and drain regions, is relatively short, the positions of the windows for the Al contacts deviate easily in the process of forming the windows and the SiO.sub.2 film beneath the phospho-silicate glass film may be accordingly exposed. The Al contacts may be therefore so disposed as to be in direct contact with the SiO.sub.2 film. If the Al contacts are in direct contact with the SiO.sub.2 film, sodium ions (Na.sup.+) and potassium ions (K.sup.+) contained in the Al contacts penetrated into the SiO.sub.2 film so that the SiO.sub.2 film is contaminated with Na.sup.+ and K.sup.+ ions. Accordingly, the surface of the substrate is changed to n.sup.+ -type region so that the reliability of the resultant device is degraded.
Further, in the process of forming the windows for Al contacts, the positions of windows sometimes deviate to make the p-type substrate exposed. In such a case, if the Al contacts are disposed in the windows, the Al contacts may connect the n-type source or drain region with the p-type substrate to cause an inferiority in characteristic (decrease in breakdown voltage).